Distortion correction for cathode ray tubes



Oct. 14, 1941. .1. DE GIER ETAL DISTORTION CORRECTION FOR CATHODE RAY TUBES Filed April 22, 1959 INVENTOR ANA/E5 De? GIE/1 AR/ENDRI'ES KRUITHOF FRANS MICHEL PENN/NG EY ATTORNEY JOH - torting causes.

Patented oer. 14, 194i UNITED STATES. PATENT OFFICE 2.253.643 4 DISTORTION CORRECTION FOB CATHODE RAY Johannes assignors TUBES Kruithof, and Netherlands,

rieken, Eindhoven, Netherlands, aA corporation ot the Netherlands Application In April 22, 1939. Serial No. 269,370 Germany yApril 25, 1938 Claims. (Ci. Z50-157) This invention relates to cathode ray tubes and in particular to methods and means for correct-l ing distortion in optical representations produced in such. tubes under the control of electrical signals.

In cathode ray tubes different causes may be responsible for distortions of the light image produced by the cathode ray beam at the point of impact on a uorescent screen. If the beam is deflected by electrostatic deflecting devices, then one of these causes is the effect of an electric field between one of the pairs of defiecting plates on the one hand and the second pair of plates on the other hand. If one plate of a pair of deflecting plates has a xed potential relatively to the cathode and the other a iiuctuating potential such distortions are particularly noted.

If, for example, a rectangle has to be filled with the beam, then the image field may have a keystone or trapezoidal area instead of a rectan` guiar shape as a consequence of the image dis- In addition, the image may be trapezoidally distorted due to the screen of impact being inclined to the axis of the electrode system, asis sometimes the case `with tubes for image-scanning in the transmitter.

It has previously been suggested to arrange in the latter case, adjacentthe point of deflection, a magnet producing a correcting magnetic field directly behind the point of deflection of the beam of rays.

According to the invention, -a different course is adapted inorder to neutralize distortion by magnetic means'. The invention is based on recognition of the fact that in contradistinction /to the electrostaticmethod it is unfavorable to cause correcting magnetic fields to act on the beam of rays in the proximity of the deiiecting means.

Accordingly, in an arrangement embodying the invention the magnet poles that bring about the correction field are not arranged adjacent the deflecting means but adjacent the point of 'impact and remote from the pointfof deflection. The correcting fields thus act at a point where the area over which the beam is deflected has a large cross-section. The correction eld can consequently be rendered sufliciently ununiform within the region where correction should be brought about. If on the contrary a magnetic correction field is provided adjacent the point of deflection at which the area over which the beam is deflected has a small cross-section, then the said field acts on the entire beam, that is`to say 55 According to the invention,

the beam is constantly deflected by the auxiliary magnetic field and the image produced at the point of impact is displaced asa whole. As regards correction the mean intensity of the mag- 5 netic field taken over the image field is not essential but the differences in intensity are important. In the case of correction adjacent the point of deflection suiciently wide differences can only be produced by powerful magnetic fields so that in this case a substantial displacement of image has to be taken into account, that is if there is any possibility of ensuring suiiicient uniformity of the magnetic field. The invention permits of creating wide differences in intensity at a low mean intensity.

In order that the invention may be clearly understood and readily carried into effect it will now be described more fully and efficient further developments will be indicated with reference to the accompanying drawing, in which Fig. 1 '1s a diagrammatic view of a cathode ray beam comprising a magnetic arrangement according to the invention;

Fig. 2 is a plan of the image screen of a cathode ray tube towards the cathode.

The cathoderay tube has an exhausted glass container .comprising a cylindrical part l and a funnel-shaped tapered part 2. The indirectly heated cathode 3 and which, it is surrounded as Well as the anode 5 and the accelerating electrode 6 serve for the production of the cathode ray beam. The tube contains in addition two couples of deilecting plates l and 8, 9 and a fluorescent screen I0.

A metal shield Il of iron shaped similarly to the container surrounds the tube at a small intermediate space and .protects it lagainst the action of external electric or'magnetic fields.

In the use of the tube one plate of each of the 40 two couples of deecting plates is connected to the accelerating electrode 6 and the other plates have different potentials fluctuating with respect to the voltage of the electrode 6 supplied to them.

If no provision is made of any further means,`then 45 the 'deflection which is imparted to the cathode ray beam is interfered with due to the asymmetrical potential distribution so that the pattern does not correspond exactly to the voltages supplied to the plates.

50 ages are such that a right-angled area is to appear on the fluorescent screen of the tube, then the result is that a trapezoidal area is recorded instead of a rectangle. In Fig. 2 this is designated by ABCD.

magnet poles l2, I3e

the control electrode l by If, for example, the deecting voit- -of impact,

target electrode, as for example, a fluorescent screen, said poles producing -ununiform magnetic elds. Thesefmagnetic ilelds are so directed and :distributed about the imag area that they exert forces on the cathode rays which bring about a correction of the distorted parts. These'forces have to be oppositely directed at the corners A and B and the magnetic field, taken over the entire image area, has to be quite ununiform. This i cannot be obtained by a correction field adjacent the point ci' deection at which the image crosssection is still very small. In order' to obtain in situ the required heterogeneous distribution there would be need for such intense ilelds that a large displacement of the beam would have to be taken into account. 'The disturbance due thereto would be even more unfavorable than the distortion to be neutralized by the magnetic ield.

The magnet poles provided in the form of construction shown are secured to the protective sheath II which serves as a magnet yoke. If

they have to serve for neutralizing a trapezoidal distortion, then two magnet poles of diierent polarity may be arranged on either side of the bases AB of the trapezium symmetrically to the centralline m. On one side of the bases are arranged the magnet poles I2 and I3 and on the other side the magnet poles I4 and l1. In many cases these four poles will be the' poles I2 and I3 or I4 and I1 respectively should exceed the distance between the poles I2 and I1 or I3 and I4 respectively, in order that the base AB should not be curved. The eld between the poles I2 and I1 and the the poles I3 and I 4 should preferably be substantially more intense than the field between I2 .and I3 and between I 4 and I1 which should exercise a minimum influence.

'I'he correction can be further improved by arranging for the short parallel sides of the trapezium CD being enclosed between four magnet poles. In Fig. 2 they are thereforethe poles I4, I5, I6 and I1. The inserted curves indicate'.

the course of a line of force and the forces exerted by the magnetic'eld on the electrons are represented in the figure by arrows. It is found that due to these forces the upright sides of the trapezium undergo slight rotation about their centre so that a rectangle is formed. No magnetic forces should act sides AB and CD. In order to render ineective any forces that may still exist at these points, the magnet poles I4 and I1 may be slightly displaced towards the other poles in the direction of the axis as shown in the drawing, so that the magnetic field-intensity gets a component in the direction of the rays which cannot bring about any deection of rays, while the component directed transversely to the direction of rays is decreased and may even become equal to zero.

An important advantage of the invention resides in that for deecting voltages uctuating symmetrically to the anode voltage the same tubes may be used as for asymmetrically uctuating deiiection voltages. There is\on1y need for the tube container being provided with a suitable set of magnet poles. l

Having described our invention, whatwe claim 1. In a cathode ray tube wherein is provided means for deecting a beam of electrons over a suillcient, but it is, however, preferable that the distance betweenfield between centrally of the uprightv stantially independent of and trons is deflected and having distortion which comprises producing a plurality of magnetic iields immediately adjacent thetarget electrode, said fields being substantially independent of and nonreacting with said deilecting means. and regulating the intensity of the produced magnetic iields to provide a nonuniform eld distribution over a surface immediately adjacent electrode.

2. In a cathode ray tube wherein is provided means for defiecting a beam of electrons over a target electrode, the method of reducing keystone distortion which comprises producing a ilrst pair of magnetic fields of predetermined intensity immediately adjacent the target electrode, produc.- ing a second pair of magnetic fields whose lntensity is different from the predetermined intensity of the first pair of produced magnetic fields, said produced second pair of magnetic iields being immediately adjacent the target electrode. both of said pairs of fields being subnon-reacting with said deecting means, the components of all of said magnetic elds over a surface equidistant from the target electrode having a value substantially equal to zero along a line of symmetry normal to two opposite sides of the area of the target electrode over which the beam of elecan intensity distribution complementary to the eld intensity disf tribution producing the keystone distortion, whereby the area of the target electrode over which the beam of electrons is deflected is ren- `dered rectangular.

3. A cathode ray distortion correcting system comprising a cathode ray tube having means to generate a beam therein, a target electrode, and deect the beam over the area of the target electrode, means for producing a plurality of electro-magnetic fields immediately adjacent the target electrode, said fields being substantially independent of and non-reacting with said deiiecting means, and means for regulating the intensity of the produced magnetic fields to provide a non-uniform neld distribution over a surface immediately adjacent and equidistant from the target electrode.

4. A cathode ray distortion correcting system coinp ing a cathode ray tube having means to generate a beam therein, a target electrode, and

meansl to deflect the beam over the area of the whose intensity is different from the predeter--Iv substantially independent of and non-reacting with said deilecting means, the components of all of said magnetic i'lelds over a surface equicathode ray .get electrode, the method of reducing keystone distant from the target ogeneous distribution electrode having a heterwhose field intensity is complementary to the field intensity producing the keystone distortion whereby the area of the target electrode over which the beam is deected is rendered rectangular.

5.A cathode ray tube for deflecting the beam oi' electrons over the area of the target electrode, a magnetic shield surrounding the tube, a plurality of magnetic pole and equidistant from the target pieces diametrically opposed mounted upon the interior of the shield and longitudinally displaced from the target electrode, and a further plurality of magnetic poles mounted on the magnetic shield and positioned intermediate the deecting means and target electrode and adjacent thereto, said pole pieces and poles having such longitudinal displacement from said va ilecting means as to be substantially independent of and non-reacting with said dellecting means.

JOHANNES nl: GmR. ARIE ANDRIES KRUn'HoF. FRANS MICHEL PENNING. 

